Automatic volume control circuit



March 4, 1941.

C. N. KIMBALL AUTOMATIC VOLUME CONTROL CIRCUIT Filed Dec. 16, 19357 7'0 A/EXTLFSTAGE 7'0 IOURCE OFLF.

S/G/VAZS' 2' Sheets-Sheet 1 mmvm JI-"IVVERTFR N y In Q/ ENVENTOR N CHARLES Al. K/MBALL ATTO R N EY Patented Mar. 4, 1941 UNITED STATES AUTOMATIC VOLUME CGNTROL CIRCUIT Charles N. Kimball, East Orange; N. J assignor to Radio Corporation of America, a. corporation of Delaware Application December 16, 1937, Serial No. 180,093

11 Claims.

My present invention relates to gain control circuits for radio receiving systems, and. more particularly to automatic volume control circuits whose operation depends upon degenerative action.

One of the main object-s of my present invention is to provide a method of controlling the signal transmission between a source of signal energy and a signal utilization network, the meth- 0d including the impression of the signals upon an impedance disposed in the space current path of an electron discharge tube; the utilization network deriving its signals. from the impedance; and there being developed a voltage across the impedance in polari-tyopposition to the signals impressed. thereon in response to an increase in signal carrier amplitude. Another important object of my invention is to provide an automatic volume control circuit 2 whose action depends upon the voltage developed across an impedance in response to the amplitude of the received signals; the impedance being arranged to receive signals from asignal collector, and the voltage; developed across the impedance and the signals impressed thereon being combined in polarity opposition and transferred to Gil a signal transmission tube. coupled to the impedance.

Another object of the invention may be stated to reside in the provision of aradio receiving system of the type-comprising a signal source followed by an implifier. having a tuned input circuit; a tube having an impedance in its space current path being arranged to have signals from the source impressed across the impedance, and

a the. amplifier deriving. its signals from the last named impedance; the input electrodes of the said tube being connected across the impedance in such a manner that an increase in gain of the km tube causes a voltage to be developed across the impedance in degenerative hase to the signals impressed thereon by the signal source, and the gain of the tube being regulated by the received carrier amplitude.

Still other-objects. of my invention are to improve genera-11y the efficiency of automatic volume control circuits for radio receivers, and more especially to provide an automatic volume control circuit which utilizes degenerative action, and which circuit is not only reliable in operation, but is economically manufactured and assembled in a radio receiver.

The novel features which I believe to be characteristic of. my invention are set forth in particularity in vthe appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings, in which I have indicated diagrammatically several circuit organizations whereby my inventionmay be carried into effect.

In the drawings: I Fig. 1 shows an illustrative circuit arrangement embodying the invention,

Fig. 2' schematically shows a receiving system employing a. modification of the invention, and i Fig. 3 is a circuit diagram of a receiving sys-' tem' employing a preferred embodiment of the invention.

The present invention is explained in Fig. 1 in a purely illustrative manner, wherein there is shown generally an amplifier tube l of the type having a high plate resistance; such a tube may be, for example, of the pentode type. Let it be assumed that the amplifier is used in a superheterodyne receiver, and that its input electrodes are coupled across the tuned secondary circuit 2 of the preceding 1-. F. transformer 3. The tuned primary circuits may-be connected to the output 2 circuit of the first detector, or to the plate circuit of a preceding I.-F. amplifier. The plate circuit of the amplifier tube l includes a resonant circuit comprising the condenser 4 and coil. 5, the circuit being fixedly resonant to the operating I. F., and the latter may have a frequency value chosen from a range of '75 to 45.0 kc. The signal voltage developed across circuit-s 45 is imp es d across an inductive impedance 6 which is disposed in the grounded cathode circuit of an electron discharge tube 1.

The tube 7 may be of the 6L7 type employing two control grids or it may be a 6.77, the control grid 8 adjacent the cathode 9 being connected .to the grounded end of the coil .6, through a negative biasing source 8. The cathode end of coil 6 is connected through condenser ID to the plate endpof coil 5, whereas the condenser H connects the grounded end of coil 6 to the low potential terminal of coil 5. The signal vol-tage developed across the I. F. circuit 4- 5 is impressed across the coil 6 by the condensers l0 and ,l l. The resonant circuit l2, fixedly tuned to the operating I. F., is coupled to the coil Bsothat the circuit l2 has developed. across it alternating voltage from coil 6. The voltage developed across'coil 6 is the resultant of the signal voltage impressed thereacross through condensers Illv and H and the voltage developed by virtue of the space current flow of tube 1. The I. F. circuit 12 may feed one, or more, I. F. amplifiers, and the latter may be followed by the customary second detector and audio network of the receiver.

The space current flow through tube 1 is varied by con-trolling the voltage of the control grid I3 which might be the third grid of a 6L7 disposed between the plate of tube 1 and its control grid 8, or the first grid of a 6L7. The grid I3 is connected by lead I4 to the cathode side of a load resistor I5; the latter resistor being connected in series in a path which includes a diode I6 and a resonant circuit II. The resonant circuit I1 is reactively coupled to the I. F. circuit 4-5 so that I. F. energy may be transferred to the circuit I! for rectification by diode I6. The circuit I! is fixedly tuned to the operating I. F., and the rectified current flowing through resistor I5 develops a voltage thereacross which'is employed to control the voltage of the grid I3. The condenser I8 is connected in shunt with load resistor l5, and functions to by-pa-ss I. F. components of the rectified current. The audio filter elements- I4 are employed to isolate grid I3 from the detector I6 for audio currents} The anode side of resistor I5 is connected to a point on the power supply bleeder resistor P, and this point is negative with respect to the ground point. Of course,;the plates of tubes I and I may be connected to proper positive potential points on the bleeder resistor P.

To explain the operation of the arrangement shown in Fig. 1, let it be assumed that a signal voltage is applied between the input electrodes of amplifier tube I. Then, the voltagedeveloped across theI. F. circuit 4-5 is equal to the in put voltage of tube I multiplied by the product of the mutual conductance of the tube and the tuned impedance of circuit 4-5 in shunt with the oathode to ground impedance of tube 1; the tuned impedance is equal to the inductance of circuit 4 -5 divided by the product of the capacity there-l of and its resistance. In the absence of signals, the gain of tube I is zero; this being accomplished by connecting the control grid I3, through lead I5 and resistor I5, to a point on the bleeder resistor P such that the'tube is biased substantially to cut-01f. In this condition of no-signal reception, the amplified voltage developedacross .I. F. circuit 4-5 is impressed across the coil 6,

and the amplified voltage is, in turn, transmitted to the following tuned circuit l2. This is the condition of maximum signal voltagetransfer, and is, of course, desirable for weak signal reception.

- However, as the signal carrier amplitude increases, the direct current voltage developed across load resistor I5 increases, with the result that the direct current voltage of control grid I3 becomes less negative thereby increasing'the gain of tube 1. If the gain of tube v'lbecame unity, the net alternating voltage developed across coil 6 would be zero as a limit. It can readily be shown that the gain of tube 1 cannot'reach unity, because of the degenerative connection between control grid 8 and the grounded end of coil 6. However, unity gain is approached'when the impedance in the cathode circuit of tube 1 is large compared to the plate resistance of the tube. as output from coil 6 is from a value e, with the tube 1 cut off and voltage measured between cathode and ground, to

Mi-L

The range of signal voltage available with tube 1 running at maximum possible gain.

It will now be seen that as the cathode end of load resistor I5 becomes more positive, the gain of tube 1 will increase thereby increasing the space current flow through coil 6. This, in turn, causes an alternating voltage of increasing value to be developed across the coil which is in degenerative phase with the signal voltage developed thereacross by virtue ofthe signal transfer from circuit 4-5. Hence, as the gain of tube I increases, the resultant signal voltage transfer to circuit I2 decreases; and at a rate such that the signal carrier amplitude at the second detector input circuit will be substantially uniform. I

The circuit arrangement of Fig. 1, also, has an automatic band-widening action; that is, an automatic selectivity control operation occurs due to the fact that the impedance between the cathode of tube 1 and ground decreases as the gain of the tube increases. Accordingly, with strong signals as when receiving local stations, the selectivity of the primary circuit 4-5 is-low, with weak, or distant, signal reception, the selectivity of circuits 4-5 is high because the tube I is biased near cut-off. The impedance seen by the tuned circuit 4-5 due to the presence of tube 1 is very nearlythe reciprocal of the mutual a modification of the present invention. The

circuit shown is of the superheterodyne type, and comprises a signal collector 20 which may be of the grounded antenna circuit type. The collector circuit 20 includes a coil 2| which is reactively coupled to the coil 22 connected between the cathode of tube I and ground. It will be understood that the tube 1' corresponds to the tube I of Fig. 1; the control grid 8' is connected to the grounded end of coil 22 through bias source 22'. The tube I is shown as of the type wherein the grid I3 is shielded from grid 8' by a positive screen. The alternating voltage across coil 22 is transferred to the tunable input circuit of the first radio frequency amplifier 30. The tunable input circuit of amplifier 30 comprises a coil 3| having the high potentialend thereof connected to the control grid of tube 30. Between the coil 3| and ground there is connected a link coupling coil 32 which is reactively coupled to the coil 22. The coils 2|, 22 and 32 are mutually coupled. The networks following the tunable amplifier 30 are conventional. They comprise a converter having a tunable signal input circuit 40 and a tunable local oscillator tank circuit M. The dotted line 42 denotes the uni-control tuning mechanism which varies the rotors of the variable tuning condensers. The I. F. energy output of the converter network is transmitted to an I. F. 'amplifier'50 through an I. F. transformer, and the amplified I. F. energy is impressed upon a diode rectifier 60 having an I. F. tuned input circuit 6| of the load resistor 63 is connected by lead 64 to v the control'grid I3 of tube 1'. The audio filter- I34 is included in lead 64. Theanodesideof rectifier 60 is connected by lead I0 to a point on the power supply bleeder resistor P, which point is at a negative potential with respect to an intermediate grounded point of the bleeder resistor.

In the absence of signals the bias on the grid l3 of tube 1' is such that the tube 1' is biased to plate current cut-off. In that case the entire signal carrier voltage developed across coil 2| is transferred to coil 32. When the control tube 1 has its gain increased by virtue of increase in the direct current voltage across load resistor 63, then the net alternating current voltage developed across coil 22 is equal to the difference between the signal voltage across coil 2| and the degenerative voltage thereacross. It will be seen that when the gain of tube 1 varies with signal carrier amplitude so as to increase with an increase in signal amplitude, then as the gain of tube 1 approaches unity, the alternating current voltage transmitted to coil 32 decreases. In this way it is possible to' maintain the carrier amplitude at the demodulator input circuit substantially uniform over a wide range of variation of the antenna signal voltage.

Cross-talk is small in the control tube I, be cause the net grid-cathode voltage is relatively small even at high signal levels. The advantage of using the link coupling coil 32 is that it prevents the necessity of directly connecting the output impedance of control tube 1' to the tunable input circuit of the amplifier 30. Since the control tube 1 has a low output impedance, it would dull the selectivity of the tunable input circuit if such direct connection were used. It may be necessary in some cases to apply in a slight degree the conventional A. V. C. arrangement of controlling the gain of tubes following the amplifier 30 in order to keep the signal to noise ratio within allowable limits. Where it is desired to prevent the hiss in subsequent transmission tubes from attaining an undesired level, A. V. C. action may be supplied by an additional rectifier which has an input circuit 8! coupled to the I. F. amplifier output circuit 62. In that case, the load resistor 82 develops A. V. C. bias which is applied to the control grid circuit of I. F. amplifier 50, and the audio voltage component across .resistor 82 may be used in the following audio network.

The antenna 20, acting as a source of voltage, will have an appreciable impedance. Hence, the voltage supplied to coil 22 is not constant, but is dependent on the internal impedance seen between the cathode of tube 1 and ground. It can be shown that the effect of increasing the gain of tube 1' is to increase the damping of the antenna circuit. Hence, the A. V. C. action involves a reduction in the magnitude of signal voltage fed to amplifier 30; the reduction depending on both degenerative voltage being produced across coil 22, and damping of the antenna circuit. In Fig. 1 a similar damping action is secured. since the circuit feeding coil 6 has a high impedance.

Best results, from. cross-talk and signal to noise ratio aspects, are secured by employing the invention in the antenna circuit for cross-talk decrease, and in the first detector output to hold down the noise amplification. Thus, in Fig. 3 there is shown a converter network employing a tube 80; its signal grid BI is coupled to the tunable circuit 82, while local oscillations, from any desired source, are impressed on the grid 83. The converter may be of the combined local oscillator-first detector type; or it may utilize a separate tunable oscillator tube to feed local oscillations to grid 83.

The signal input circuit 82 comprises the variable tuning condenser 84 and the coil 85. The antenna 86 is connected to the high potential side of circuit 82; the cathode 81 of tube 88 is connected to the same side of the circuit. The control grid 89 is established for alternating current potential, at the potential of the grounded end of the circuit 82; a condenser 98, of low impedance to high frequency currents, connects grid 89 to the grounded end of coil 85. The plate of the tube 83 is connected to any proper positive potential point. The signal voltage developed across cail is transferred to the input electrodes of tube 3%). The usual self-biasing network Si is disposed in the grounded cathode lead of tube 85. The grid 8| is coupled by condenser 92 to the cathode end of coil 85, and a leak resister 93 connects the grid 81 to. ground. It wi be understood that condenser $4 will be uni-controlled with the local oscillator condenser, as shown in Fig. 2.

The I. F. output of converter as is transmitted to amplifier 94 by the transformer 95 which has its primary and secondary circuits each resonated to the operating I. F. value. The effective signal voltage developed across the secondary circuit is controlled by tube 96; its functioning is similar to that of tube 88. The cathode of tube 96 is connected to the high potential end of the secondary coil ill; the grounded end of the coil is connected to the grid 99 by the I. F. bypass condenser 9s. The amplified output of tube 94 is transmitted to the input electrodes of the de tector tube ml by the I. F.-tuned transformer [ML The detector Jill is of the infinite impedance diode detector type. Such a detector has, also, been referred to as a degenerative plate circuit detector, because there is a degenerative audio voltage produced across the cathode load resistor I 92.

The condenser I93 in shunt with resistor Hi2 bypasses I. F. currents, but not audio currents.

Normally, that is in the absence of received signals, there is a s .1fficient voltage drop across resistor '92 to bias the control grid of tube Hil close to plate current cut-off. Hence, with increasing signal amplitude, the cathode side of resistor m2 becomes increasingly positive in direct current voltage with respect to ground. It is not believed necessary further to describe the functioning of the detector circuit, since it has been disclosed and claimed by P. O. Farnham in application Serial No. 8,864, filed March 1 1935. It is sufficient to point out at this time that the voltage across resistor m2 bears a substantially linear relation to the I. F. voltage transmitted through transformer use.

The audio .voltage developed across resistor 582 is transmitted to any desired audio utilization network. The grid end of resistor 982 is connected to a point on the voltage supply resistor Hi4, and which point is sufiiciently negative with respect to an intermediate grounded point so that the control grids of regulator tubes as and on are biased to plate current cut-off. To accomplish this each of control grids 89 and 99 are connected by direct current voltage connections to the negative point Hi5 on the voltage supply resistor H34. Thus, the control grid 89 is connected to the negative point we through a path which includes the audio filter 5%, lead Hi'l, resistor Hi2. Control grid 99 cf regulator tube 96 is connected to negative point Hi5 through the cathode end of resistor I92 becomes increas-' ingly positive, and renders grids 89 and 99 less negative. This action, in turn, causes the gain of each of tubes 88 and 96 to increase. As a consequence, there is developed across each of coils 85 and 9? alternating current voltage which is in degenerative phase with the signal voltage impressed upon these coils. In this way, the carrier level at the second detector input circuit I is maintained substantially uniform in spite of carrier amplitude variations at the antenna 86. The control action of tubes 88 and 96 is the same as described heretofore in connection with Figs. 1 and 2.

In addition, however, it will be noted that the control tubes 88 and 96 function to introduce damping of the antenna circuit 86 and transformer 95 respectively. Again, since the low impedance regulator tubes 88 and 96 shunt the radio frequency and. intermediate frequency tuned circuits, their maximum shunting action occurs with strong signals; that is, when the regulator tubes have maximum gain. Hence, adverse selectivity effects in the I. F. circuit are not objectionable, since it is in the correct control sense. Further, reduced image rejection in 0 the radio frequency circuit is not objectionable for strong signal reception. If desired, the second regulator tube 96 can be arranged so that its grid 99 receives less positive voltage from resistor m2 and the control action is graduated. While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a source of signals, a

space discharge tube having an input electrode,

a plate and a cathode, an impedance disposed in the space current path of the tube and having one end thereof connected to the cathode, and said input electrode being connected-to a point on said impedance which is in phase opposition to the cathode end of said impedance, means for transferring signals from said source to the impedance, a wave utilizing network coupled to said impedance to receive alternating voltage developed across the impedance, solely a direct current source connected to said plate, and rectifier means responsive to signal amplitude variation for controlling the gain of said tube thereby to control the transfer of signal energy from said source to said utilizing network.

2. In combination with a source of signals, a space discharge tube having a pair of input electrodes, an impedance disposed in the space current path of the tube, and said input electrodes being connected to points of different potentialon said impedance, means for transferring signals from said source toipointsnof different potential on the impedance, a waveutilizing network coupled to said impedance to receive alternating voltage developed across the impedance, and rectifier means external of said tube responsive to signal amplitude variation for controlling the gain of said tube, said input electrodes of said tube consisting of a control grid and a cathode, said impedance being connected between said cathode and a point of fixed potential, and said control grid being connected to a point on said impedance which is opposite in alternating current polarity with respect to the cathode as space current fiows through the impedance.

3. In combination with an antenna circuit of a radio receiver and the turnable input circuit of the first radio frequency amplifier tube, a control tube provided with at least a cathode, control grid and an output electrode, an impedance disposed in the space current path of the tube and connected between said cathode and a point of fixed potential, the control grid of said tube being connected to a point on the impedance which assumes an alternating potential out of phase with respect tothe cathode potential as the space current flow of said tube increases, said antenna circuit being coupled to said impedance, said tuned circuit being coupled to said impedance, said'output electrode being free of any connection to said input circuit and being connected to solely a source of direct current whereby the space current of said control tube flows through only said impedance, and means responsive to carrier amplitude increase for increasing the gain of said tube.

4. In combination with a signal transmission tube having a tuned output circuit, a following tuned network, a control tube provided with a cathode, a control grid. and a plate, said control tube having an inductive impedance connected between the cathode thereof and ground, said following network being reactively coupled to said impedance, means impressing signals across said impedance, said control tube including a control grid connected to a point on said impedance which is in polarity opposition for the signal frequency voltage with respect to the cathode, said plate being connected to solely a direct current source whereby the space current of the control tube flows through only said impedance, and means responsive to signal carrier amplitude variation for controlling the gain of said control tube.

5. In combination with a pair of cascaded signal transmission tubes, a tuned signal input circuit coupled to the first of the tubes, an antenna circuit feeding signals to said input circuit, a regulator tube including in its cathode circuit an inductive impedance which is part of the said tuned input circuit, means connecting a control grid of the regulator tube to a point on the impedance which is in phase opposition to the cathode end of the impedance, and rectifier means, responsive to .an increase in the intensity of the signal output of the second transmission tube, for increasing the gain of the regulator tube thereby to decrease the signal voltage magnitude at the said first tube.

6. In combination with a pair of cascaded signal transmission tubes, a signal input circuit ductive impedance which is part of the input circuit, means connecting a control grid of the regulator tube to a point on the impedance which is in phase opposition to the cathode end of the impedance, means, responsive to an increase in the intensity of the signal output of the second transmission tube, for increasing the gain of the regulator tube thereby to decrease the signal voltage magnitude at the said first tube, means deriving an alternating voltage from the output of the first transmission tube which is in phase opposition to the signal voltage, means combining the saidderived and signal voltages, and means responsive to the gain increasing means for controlling the magnitude of the derived voltage.

7. In combination with a pair of cascaded signal transmission tubes, a signal input circuit coupled to the first of the tubes, an antenna circuit feeding signals to said input circuit, a regulator tube including in its cathode circuit an inductive impedance which is part of the input circuit, means connecting a control grid of the regulator tube to a point on the impedance which is in phase opposition to the cathode end of the impedance, and means, responsive to an increase in the intensity of the signal output of the second transmission tube, for increasing the gain of the regulator tube thereby to decrease the signal voltage magnitude at the said first tube, said gain increasing means comprising a detector having an input circuit coupled to the second transmission tube output circuit, and a gain control connection from the regulator tube to a point in the detector which increases in a positive polarity sense as the signal intensity increases.

8. In omb-ination with a signal transmission tube having input and output electrodes, a signal source, a reactive impedance coupled between the input electrodes of said tube, one end of said 40? impedance being at a fixed potential, said source respect to alternating current potential developed across the impedance, and signal rectifier means for controlling the space current flow through the control tube thereby to vary the magnitude of signal energy transferred from said source to the input electrodes of said transmission tube.

9. In combination with a signal transmission tube having input andoutput electrodes, a signal source, a reactive impedance coupled between the input electrodes of said tube, one end of said impedance being at a fixed potential, said source being coupled to the impedance to impress signal energy thereupon, a control tube provided with at least a. cathode, control grid and plate, said cathode being connected to the opposite end of the said impedance, said grid being established at said fixed potential end, said plate being free of any connection to said impedance and being connected to solely a source of direct current whereby the space current of the control tube flows through said impedance, said two ends of the impedance being in phase opposition with respect to alternating current potential developed across the impedance, and signal rectifier means responsive to variations in signal amplitude at said source for controlling the space current fiow through the control tube thereby to vary the magnitude of signal energy transferred from said source to the input electrodes of said transmission tube.

10. In combination with a signal transmission tube having input and output electrodes, a signal source, a reactive impedance coupled between the input electrodes of said tube, one end of said impedance being at a fixed potential, said source being coupled to the impedance to impress signal energy thereupon, a control tube provided with at least a cathode, control grid and plate, said cathode being connected to the opposite end of the said impedance, said grid being established at said fixed potential end, said plate being free of any connection to said impedance and being connected to solely a source of direct current whereby the space current of the control tube flows through said impedance, said two ends of the impedance being in phase opposition with respect to alternating current potential developed across the impedance, and means for controlling the space current fiow through the control tube thereby to vary the magnitude of signal energy transferred from said source to the input electrodes of said transmission tube, said reactive impedance being a circuit tuned to a desired signal frequency, and said last control means including a detection device coupled to said transmission tube output electrodes.

11. In combination with a signal transmission tube having input and output electrodes, 2. signal source, a reactive impedance coupled between the input electrodes of said tube, one end of said impedance being at a fixed potential, said source being coupled to the impedance toimpress signal energy thereupon, a control tube provided with at least a cathode, control grid and plate, said cathode being connected to the opposite end of the said impedance, said grid being established at said fixed potential end, said plate being connected to a source of direct current whereby the space current of the control tube flows through said impedance, said two ends of the impedance being in phase opposition with respect to alternating current potential developed across the impedance, and means for controlling the space current flow through the control tube thereby to vary the magnitude of signal energy transferred from said source to the input electrodes of said transmission tube, a detector of the degenerative plate circuit rectification type having its input coupled to said transmission tube output electrodes, and said control means including a load resistor in the detector space current path having a direct current voltage connection to said control tube.

CHARLES N. KIMBALL. 

